1. Field of the Invention
The present invention relates to a lithography using an X-ray beam and, more particularly, an X-ray lithography system having a distortion correction (or magnification correction) function in pattern projection and an X-ray lithography method using the same.
2. Description of the Related Art
In recent years, the level of miniaturization and high accuracy of the design rule for the semiconductor device has been constantly risen. In the optical lithography, the advanced technology such as resolution enhancement technology using a phase-shift mask and an excimer laser has been proposed. However, such request has become more severe, and essential limitation because of the wavelength of light has become apparent. In particular, the severe request for dimensional standard (short dimension (critical dimension)/long dimension accuracy) of the exposure mask and alignment accuracy has been made. Such request has been similarly made on the X-ray lithography which is promising as one of the advanced lithography technologies from respects of resolution and throughput in the exposure technology of less than 0.15 .mu.m. In the X-ray lithography, the reduction printing system has not been at the practical use level at present since normally it is difficult to implement the efficient refractive or reflective optical system, and therefore the one-by-one exposure system has been mainly developed. In the one-by-one X-ray lithography, in order to projection the pattern formed on the mask by the proximity exposure method onto the wafer at substantially 1.times. magnification, improvement of the critical dimension/long dimension accuracy of the pattern on the mask, improvement of the high resolution pattern projection technology, improvement of the pattern alignment accuracy between the mask and the wafer and the process substrates, etc. have been important development items. For this reason, especially various studies concerning the improvement of the critical dimension/long dimension accuracy of the X-ray mask pattern as the key technology have been done in the prior art. For example, reduction in stress of the X-ray absorber pattern and the membrane (the X-ray transmissive thin film), uniformization of stress and film thickness, improvement of the pattern delineation accuracy, and development of the etching technology have been advanced. However, if the lithography means of less than 0.15 .mu.m is taken into account, the mask having accuracy at the practical level has not been fabricated yet and also the lithography system comprising the exposure system has not constructed yet. Meanwhile, not only the accuracy but also the function for accommodating sufficiently the existing lithography process have been requested in the advanced lithography. For example, in the manufacturing of the LSI and others, normally several tens to one hundred or more of complicated and various fabrication steps are continued to superpose ten or more sheets of fine mask patterns mutually. In such LSI manufacturing steps, there is the case the mask pattern must be superposed mutually by using different type of steppers like a combination of the optical stepper and the X-ray stepper. In the case where different type of steppers are employed, the exposure sequences such as exposure field size and direction, pre-alignment system, etc. must be matched with specifications mutually and therefore it is the premise that such exposure sequences can be practically used freely in combination according to various steppers. In particular, normally the image distortion of the exposure field due to lens distortion, difference in the magnification in the manufacturing process, etc. are contained in the substrate which is formed by the optical exposure system as the conventional lithography. In order to superpose the overlying circuit patterns onto the substrate on which the underlying circuit patterns are formed by the conventional lithography within predetermined accuracy, the exposure function which can correct correctable distortions as much as possible are needed at the lowest. Such correcting function is of course indispensable for the exposure method employed in the X-ray lithography, nevertheless the correcting functions being studied up to now in the prior art have not reached the practical level yet in the existing state. Up to now, as a means for attaining projection accuracy according to a correction function, the methods physically expanding/contracting or deforming a part or all of the mask or the wafer are proposed. However, there has been a possibility that complexity and high cost in the mask manufacturing process are caused by these proposed method. Furthermore, there has been another problem that complexity of the stage mechanism of the exposure system is caused. Also, such systems are difficult to operate at high speed because physical deformation is applied, so that the one skilled in the art has been anxious about the problems of reduction in throughput and mask endurance (fatigue fracture).
In the conventional proposals concerning the beam line and the X-ray reflection mirror in the X-ray lithography system, most of them relate to rocking and drive of the mirror and the mirror shape in order to improve uniformity of the exposure intensity and to expand the exposure region. The proposals concerning the mirror shape are limited to batch expansion of the exposure region and improvement of uniformity of the exposure intensity. Other proposals is mainly concerned with the technology of reducing the divergence of the X-ray beam and enhancement of the collimation of the X-ray beam. In Patent Application Publication (KOKAI) Hei 7-78755, the exposure system having a two-sheeted X-ray reflection mirror construction, in which the vertical phase error of the exposure field can be varied by changing a grazing angle of the second X-ray reflection mirror during shifting the second X-ray reflection mirror linearly, has been proposed.
The X-ray exposure technology in the prior art has been achieving, to same extent, expansion of the exposure field as well as maintenance of uniformity of the exposure dose for the present. However, since the pattern formed on the X-ray mask once is exposed still remaining positional distortion caused at the time of mask fabrication, there has been the problem that alignment accuracy between the X-ray mask and the wafer cannot be sufficiently obtained. Even when the X-ray mask pattern can be formed with fully desired accuracy and the exposure is effected by using the X-ray beam having extremely high collimation, the alignment accuracy of the overlying pattern to mate with the X-ray mask pattern has not been able to be achieved in fact if the underlying exposure field profiles are different every semiconductor substrate (semiconductor chip) because of difference in the LSI manufacturing process. Also, in the technology disclosed by Patent Application Publication (KOKAI) Hei 7-78755, there has been such a problem that, although the positional distortion in the vertical direction of the exposure field can be reduced in exposure by applying the exposure system disclosed by this Publication, the distortion in the horizontal direction of the exposure field cannot be reduced. In addition, it has been impossible to improve complicated positional distortion such as perpendicularity correction, etc.
As described above, it has been impossible for the X-ray exposure technology in the prior art to reduce or correct the positional distortion simultaneously in both the horizontal and vertical directions in exposure. For example, as mentioned above, the desired alignment accuracy has not been able to be attained for various substrate or chips unless the pattern position and the exposure field profile are changed by applying physical action to the X-ray mask. In addition, the X-ray beam has a certain finite divergence angle since such X-ray beam is not perfectly in parallel, but the problem of image distortion caused by the divergence angle has arisen since the divergence angle is a fixed value. At the present, the method which corrects the projection displacement generated by the divergence of the X-ray beam previously in depicting and forming the mask pattern has been proposed as the method of dissolving the above problem. However, according to the method in which the X-ray mask pattern is designed and formed every time while correcting the image distortion due to the manufacturing process or the divergence of the X-ray in accordance with various LSI substrates respectively, there has been the problem that such method cannot be applied individually in case the LSI manufacturing process is changed or in case the X-ray exposure system or the beam line is changed.
In Patent Application Publication (KOKAI) Hei 8-55785, the overall magnification correction can be performed by using proximity gap change and temperature control and the magnification correction along the scanning direction can be performed by minutely moving the semiconductor wafer and the X-ray mask relatively in synchronous with the scanning of the X-ray beam, so that the projection rate correction can be implemented independently in the longitudinal direction and the lateral direction. According to this method, the magnification correction can be done in the horizontal direction and the vertical direction separately. However, as described above, since the field size must be changed by expanding/contracting the substrate by virtue of temperature control according to each substrate or chip or the gap must be changed if the underlying exposure field profile is different every substrate or chip according to difference in the process, there has been the problem that the much process time is consumed and the process speed is decreased. As with the substrate contraction due to heat, it has become difficult to control a heat source and keep the exposure atmosphere at a constant temperature. In the method of changing the gap, there has been the problem that, since pressure difference is caused between the upper surface and the lower surface of the X-ray mask as a thin film structure at the time of changing the gap and thus the Membrane of the mask is distorted mechanically (physically), it is difficult to control such gap.
Furthermore, there has been the problem that, if the mask itself is deformed due to heat generated by the exposure, vibration, variation of the gap, etc., the projection of the pattern to mate with the underlying profile with predetermined positional accuracy is difficult from the aspect of responsibility.